Compound semiconductor and method for manufacturing same

ABSTRACT

A nitride compound semiconductor having a low resistivity that is conventionally difficult to be manufactured is provided. Since the nitride compound semiconductor exhibits a high electron mobility, a high-performance semiconductor device may be configured. The present invention may provide, at a high productivity, a group 13 nitride semiconductor of an n-type conductivity that may be formed as a film on a substrate having a large area size and has a mobility of 70 to 140 cm2/(V·s) by a pulsed sputtering method performed in a process atmosphere at room temperature to 700° C.

TECHNICAL FIELD

The present invention relates to a compound semiconductor and a methodfor manufacturing the same.

BACKGROUND ART

Devices using group 13 nitride semiconductors such as GaN and InN arenow put into practical use in a wide range of products. Conventionally,an MOCVD method and an MBE method have been used for crystal growth ofsuch a group 13 nitride semiconductor. However, the MOCVD methodrequires a process temperature exceeding 1000° C. The MBE method allowsa compound semiconductor film to be formed at a low temperature, but isnot suitable to mass manufacturing because there is a limit on the areasize of the film that may be formed and the manufacturing cost is high.

With the MBE method, if donors are incorporated at a high concentration,absorption by the high concentration donor level generated in theforbidden band in the vicinity of the conduction band of the crystalstructure occurs. For this reason, the MBE method has a problem that thetransparency of the manufactured compound semiconductor film isdecreased. For these reasons, the MOCVD method is now used tomanufacture a compound semiconductor, mainly, to manufacture a nitridesemiconductor for a practical use (Non-patent Document 1).

Currently, next-generation electronic devices having both a highwithstand voltage and a low on-resistance are desired. In order torealize such an electronic device, it is desired to realize a two-,three-, or four-component compound semiconductor, more specifically, acompound semiconductor device using a group 13 nitride semiconductor.This requires further improvement in the quality of the crystal of sucha compound semiconductor and improvement in the refinement of the dopingtechnology. Especially for a vertical power device to be formed on a GaNsubstrate, it is urged to decrease the carbon concentration of an n-typedrift layer and to improve the electron mobility. There are thefollowing documents describing the prior art.

Patent Document 1 discloses a semiconductor device including a bufferlayer formed of a metal nitride and a semiconductor layer, which areprovided on a copper substrate.

Patent Document 2 discloses examples of a semiconductor substrateincluding a graphite plate having a thickness of 10 to 100 μm,containing a sintered polymer and having a heat resistance andflexibility, a buffer layer formed of HfN on the graphite plate, and asemiconductor layer formed of GaN on the buffer layer. Patent Document 3discloses a method for manufacturing a group-III-V compoundsemiconductor by epitaxial growth on a ZnO substrate.

Patent Document 4 and Patent Document 5, which are regarding a nitridesemiconductor, will be described in paragraph 0167 and thereafter inthis specification.

Patent Document 6 is cited as a prior art document in the InternationalSearch Report for a PCT application described below (PCT/JP2017/020513filed by the present applicant). Patent Document 6 disclosesexperimental results that indicate that even when the Si concentrationis increased up to 2×10E+20/cm³, the AlGaN film is not roughened (FIG. 4).

Now, Non-patent Document 1 described above discloses research results onformation of a p-type GaN semiconductor layer. Non-patent Document 2discloses research results on the contact resistance of a p-type GaNsemiconductor layer. Non-patent Document 3 discloses results of aresearch for producing p-type GaN, for an LED device, based on InGaN bylow-temperature growth by a PSD method. Non-patent Document 4 disclosesresearch results on the electron mobility and the doping concentrationregarding silicon.

Non-patent Document 5 discloses research results on a model of carriermobility of GaN. Non-patent Document 6 discloses research results onevaluation of the contact resistance against a p-type GaN formed by thePSD method. Non-patent Document 7 discloses experiment examples ofproducing an LED on glass. Non-patent Document 8 discloses researchresults on a nitride single crystal grown by the PSD method. Non-patentDocument 9 discloses a normally-off Ge-doped GaN transistor having anextremely low on-resistance.

Non-patent Document 10 discloses research results on Si-doped AlGaNhaving a low resistance and a high carrier concentration. Non-patentDocument 11 discloses examples of experiments performed under theconditions that the Si concentration was 2×10¹⁶ cm⁻³ and the mobilitywas 1034 cm²/(V s). Non-patent Document 12 discloses an epitaxiallygrown film of GaN doped with Ge by the PSD method.

Non-patent Document 13 discloses, in detail, various characteristics ofn-type GaN doped with Ge and Si that may provide new properties.

Finally, Non-patent Document 14 reports research results on fabricationof a high quality nitride semiconductor by a sputtering method andapplication thereof to devices.

CITATION LIST Patent Literature

-   Patent Document 1: Japanese Laid-Open Patent Publication No.    2008-243873-   Patent Document 2: WO2011/021248A1-   Patent Document 3: Japanese Laid-Open Patent Publication No.    2010-56435-   Patent Document 4: Japanese Laid-Open Patent Publication No.    2016-115931-   Patent Document 5: United States Patent Application Publication    2016/0172473-   Patent Document 6: Japanese Laid-Open Patent Publication No.    2015-149342A

Non-Patent Literature

-   Non-patent Document 1: G. T. Zhao et al., “Optical Absorption and    Photoluminescence Studies of n-type GaN” Jpn. J. Appl. Phys. 38,    L933-L995 (1999)-   Non-patent Document 2: Arakawa et al., The Japan Society of Applied    Physics, 63rd Spring Meeting, 20p-H121-8-   Non-patent Document 3: E. Nakamura et al., Appl. Phys. Lett. 104,    051121 (2014)-   Non-patent Document 4: D. M. Caughey et al.; Proc. IEEE 55, 2192    (1967)-   Non-patent Document 5: T. T, Mnatsakanov et al.; Solid-State    Electron. 47, 111 (2003)-   Non-patent Document 6: Proceedings, The Japan Society of Applied    Physics, 62nd Spring Meeting-   Non-patent Document 7: Nikkei Electronics, NE report, pp. 14-15,    Jul. 7, 2014-   Non-patent Document 8: Fujioka “Flexible Device” project research    abstracts pp. 89-94 (published on Mar. 4, 2008)-   Non-patent Document 9: A. Suzuki et al., “Extremely low    on-resistance Enhancement-mode GaN-based HFET using Ge-doped    regrowth technique” (IEDM14, pp. 275-278 (2014))-   Non-patent Document 10: Motoaki lwaya et al., “Extremely    low-resistivity and high-carrier-concentration Si-doped AlGaN with    low AlN molar fraction for improvement of wall plug efficiency of    nitride-based LED”, 2015 Conference on Lasers and Electro-Optics    Pacific Rim (Optical Society of America, 2015), paper 28C2_2-   Non-patent Document 11: Ueno et all, “Development of n-type doping    technique for GaN by pulsed sputtering”, Proceedings, The Japan    Society of Applied Physics, 77th Autumn Meeting (2016)-   Non-patent Document 12: Ueno et al., “Highly conductive Ge-doped GaN    epitaxial layers prepared by pulsed sputtering”, Applied Physics    Express 10, 101002 (2017)-   Non-patent Document 13: Ueno et al., “Electron transport properties    of Degenerate n-type GaN prepared by pulsed sputtering”, APL    MATERIALS 5, 126102 (2017)-   Non-patent Document 14: Fujioka et al., “Growth of high quality    nitride semiconductors by sputtering and its application to device    fabrication”, Oyo Buturi, Vol. 86, No. 7, pp. 576-580 (2017)

SUMMARY OF INVENTION Technical Problem

With the conventional technology, in the case where it is attempted torealize crystal growth of a group 13 nitride semiconductor by the MOCVDmethod, carbon and hydrogen contained in the material gas areincorporated into the film. This causes a problem that it is difficultto form a high quality film having a low concentration of impuritiessuch as carbon, hydrogen and the like.

In addition, in the case where it is attempted to realize crystal growthof a group 13 nitride semiconductor by the MOCVD method, it is generallydifficult that a film having a donor concentration of 5×10¹⁹ cm⁻³ orhigher exhibits an electron mobility of about 46 cm²/(V·s) or higher dueto thermodynamic restrictions. The MOCVD method is based on a chemicalreaction. Therefore, it is in fact impossible to realize crystal growthat a low temperature, and carbon and hydrogen contained in the materialgas are easily incorporated into the manufactured film.

As a crystal growth method of a nitride semiconductor replacing theMOCVD method, a pulsed sputtering deposition (PSD) method is nowproposed. It has been demonstrated that a p-type GaN thin film having alow concentration of residual hydrogen and exhibiting a high holemobility is obtained by the PSD method (Non-patent Document 2).

Currently, the MOCVD method is used to manufacture an electronic deviceand a light emitting device on a nitride semiconductor substrate forpractical use. However, it is difficult to manufacture, by the MOCVDmethod, an n-type layer having a high donor concentration which areimportant to decrease the resistance of these devices. Therefore, thereare very few reports on the characteristics of such an n-type layer.

As can be seen, it is desired to develop a group 13 nitridesemiconductor of n-type conductivity that exhibits a high electronmobility even in a region of a high donor concentration. In such asituation, it is required to realize a semiconductor material exhibitingas high an electron mobility as possible in order to achieve thepurposes of improving the performance, saving the energy, and improvingthe efficiency of electronic devices and light emitting devices.

A research group of the present inventors proceeded research anddevelopment to raise the performance of nitride semiconductors, andpublished, as achievements thereof, “Development of low temperaturefabrication process for nitride based LEDs” (The Japan Society ofApplied Physics, 60th Spring Meeting, 30a-G21-10) (see FIG. 9A and FIG.9B ), “Creation of function-expressing nanosystems by processintegration” (JST Strategic Basic Research Programs (CREST) (see FIG.10A and FIG. 10B), “Development of n-type doping technique for GaN bypulsed sputtering”, Proceedings, The Japan Society of Applied Physics,77th Autumn Meeting, 13p-A21-3 (Autumn, 2016) (Non-patent Document 11,see FIG. 11 ), and the like.

The present invention, made in light of such a problem, has an object ofeasily manufacturing and providing a two-, three- or four-componentcompound semiconductor, more specifically, a group 13 nitridesemiconductor film, of n-type conductivity that exhibits a high electronmobility even in a region of a high donor concentration.

The present inventors have disclosed a new nitride semiconductor and amethod for manufacturing the same in a PCT application (Application No.PCT/JP2017/020513, Filing Date: Jun. 1, 2017; claiming priority based onJapanese Patent Application No. 2016-169994; Publication No.WO2018/042792A1, International Publication Date: Mar. 8, 2018).

The present invention includes a part overlapping of the examples inabove-mentioned applications and also includes new examples.

Solution to Problem

In order to solve the above-described problems, the present inventionprovides [embodiment 1] to [embodiment 20] described below.

Embodiment 1

A two-, three-, or four-component compound semiconductor containingnitrogen and one element selected from the group consisting of B, Al, Gaand In, which are group 13 elements, wherein a combination of twoproperty values of electron concentration and resistivity fulfillsnumerical value conditions enclosed by four points represented by:

(a) the electron concentration of 1.8×10²⁰ cm⁻³ and the resistivity of0.25×10⁻³ Ω·cm,

(b) the electron concentration of 3.6×10²⁰ cm⁻³ and the resistivity of0.25×10⁻³ Ω·cm,

(c) the electron concentration of 6×10²⁰ cm⁻³ and the resistivity of0.15×10⁻³ Ω·cm, and

(d) the electron concentration of 3×10²⁰ cm⁻³ and the resistivity of0.15×10⁻³ Ω·cm.

Alternatively, the combination of the two property values of electronconcentration and resistivity may fulfill, instead of the numericalvalue conditions enclosed by the four points represented by (a) to (d)mentioned above, numerical value conditions enclosed by four pointsrepresented by:

(a-1) the electron concentration of 1.5×10²⁰ cm⁻³ and the resistivity of0.20×10⁻³ Ω·cm,

(b-1) the electron concentration of 6×10²⁰ cm⁻³ and the resistivity of0.20×10⁻³ Ω·cm,

(c-1) the electron concentration of 6×10²⁰ cm⁻³ and the resistivity of0.10×10⁻³ Ω·cm, and

(d-1) the electron concentration of 4×10²⁰ cm⁻³ and the resistivity of0.10×10⁻³ Ω·cm. Preferably, the resistivity is 0.18×10⁻³ Ω·cm or lowerinstead of the range having the upper limit mentioned in each of (a-1)to (d-1).

Embodiment 2

The compound semiconductor according to embodiment 1, wherein theresistivity is 0.190×10⁻³ Ω·cm or lower (limited to the case of (a) to(d)).

Embodiment 3

The compound semiconductor according to embodiment 1 or 2, furthercontaining Si.

Embodiment 4

The compound semiconductor according to embodiment 1, 2 or 3, whereinthe compound semiconductor has an RMS value, obtained by a surfaceroughness measurement by an AFM, of 1.5 nm or less.

Embodiment 5

The compound semiconductor according to embodiment 1, 2, 3 or 4, whereinthe compound semiconductor has an n-type conductivity and an electronmobility of 80 cm²/(V·s) or higher.

Embodiment 6

The compound semiconductor according to any one of embodiments 1 to 5,wherein the compound semiconductor has an n-type conductivity and anelectron mobility of 130 cm²/(V·s) or lower.

Embodiment 7

The compound semiconductor according to any one of embodiments 1 to 6,wherein the compound semiconductor contains Ga and N as main components.

Embodiment 8

The compound semiconductor according to any one of embodiments 1 to 7,wherein the compound semiconductor contains Ga as the group 13 elementand further contains Al and/or In.

Embodiment 9

The compound semiconductor according to any one of embodiments 1 to 8,wherein the compound semiconductor contains Ge.

Embodiment 10

A contact structure, comprising a conductive portion containing thecompound semiconductor according to any one of embodiments 1 to 9 and anelectrode connected with each other.

Embodiment 11

A semiconductor device, comprising the contact structure according toembodiment 10.

Embodiment 12

A transparent electrode, comprising the compound semiconductor accordingto any one of embodiments 1 to 9.

Embodiment 13

A method for manufacturing a two-, three- or four-component compoundsemiconductor containing nitrogen and one element selected from thegroup consisting of B, Al, Ga and In, which are group 13 elements, themethod comprising:

pulsed-sputtering a target metal containing at least Ga in a chamber ina process atmosphere containing noble gas, nitrogen gas and oxygen toform a film of the compound semiconductor having a resistivity of0.4×10⁻³ Ω·cm or lower with a growth rate of 450 nm/h or less.

Embodiment 14

The method for manufacturing the compound semiconductor according toembodiment 13, wherein a substrate temperature for a formation of thefilm is 700° C. or lower.

Embodiment 15

The method for manufacturing the compound semiconductor according toembodiment 13 or 14, wherein the growth rate is set to 90 to 450 nm/h.In this embodiment, it is preferred to set the growth rate to 100 to 400nm/h, and it is more preferred to set the growth rate to 180 to 370nm/h.

Embodiment 16

The method for manufacturing the compound semiconductor according toembodiment 13, 14 or 15, wherein oxygen gas is supplied to the processatmosphere.

Embodiment 17

The method for manufacturing the compound semiconductor according to anyone of embodiments 13 to 16, wherein the sputtering is performed usingan oxygen component contained in a component remaining in the chamber orusing trace amount of oxygen component contained in another material gasor the target metal, without supplying oxygen gas into the chamber.

Embodiment 18

The method for manufacturing the compound semiconductor according to anyone of embodiments 13 to 17, wherein a distance between a surface onwhich the compound semiconductor is formed and the target metal is setto 10 to 50 cm. The distance is more preferably set to 15 to 30 cm.

Embodiment 19

A sputter gun usable for the method for manufacturing the compoundsemiconductor according to any one of embodiments 13 to 18, wherein:

the target metal is provided in a head of the sputter gun, and the headis incorporated into the chamber so as to face a substrate electrode,and the head has an effective size of about 1 inch to about 4 inches.

Embodiment 20

The sputter gun according to embodiment 19, wherein the target metal hasa circular or rectangular planar shape and is mounted on the head.

Four new experiment examples according to the present invention areshown in FIG. 1 to FIG. 3 . In FIG. 1 , the relationship between thegrowth rate of GaN doped with Si (horizontal axis) and the electronconcentration of a film of the obtained compound semiconductor isplotted. FIG. 1 shows the relationship between the growth rate and theelectron concentration in a high concentration region in which theelectron concentration is 2×10²⁰ cm⁻³ or higher. FIG. 2 shows a similarrelationship except that the vertical axis represents the resistivity.

Based on these four experiment examples, the present inventors performedfitting on the lower limit of the resistivity from the scatter data onthe growth rate and the resistivity, and obtained the predicted value of0.083 mΩ·cm (see FIG. 3 ).

Assuming that the usable effective growth rate by a sputtering devicecurrently used commercially is 59 nm/h, the result of about 0.1 mΩ·cmwas obtained from the intersection of this growth rate with the fittingline. It is considered that the resistivity of 0.1 mΩ·cm is fullyfeasible by adjusting, for example, the material and the processconditions to be used, in this manner.

According to the present invention, in order to realize a film of acompound semiconductor having desired properties, various conditions maybe adjusted to gradually find a required growth rate, and such a growthrate may be set to a value suitable to mass production. For example,according to one conceivable technique, the structure of the chamber,and the shape and the locations of the electrodes are determined, andthen, the inner pressure of the chamber, the back pressure (performanceof the vacuum pump), the type of gas to be used, the flow of the gas,the control on the impurity gas, the control on the magnetic field, thepower source, the substrate temperature, the distance between the targetand the substrate, and the like, which are parameters for the filmformation operation, are optimized. The processes that may be commonlyperformed in sputtering, for example, pre-washing, drying, heating andthe like may be performed when necessary. In addition, variouscharacteristics of the formed film sample, for example, the thickness,the state (surface roughness, cross-sectional structure), the opticalcharacteristics, the conductivity, the mechanical characteristics, andthe like of the film may be evaluated at high precision. In this manner,the film formation operation according to the present invention may bemanaged properly.

FIG. 4 is a scatter graph of the resistivity and the electronconcentration of the high concentration n-type GaN doped with Ge or Sidisclosed by the present inventors in Non-patent Document 13. FIG. 5 isa scatter graph in which data in the experiment examples newly added inthis application is plotted in addition to the data in FIG. 4 . In FIG.5 , the star marks represent the results in the new experiment examples.It is understood that one of the experiment examples shows a numericalvalue equivalent to that of the previous experiment examples, whereasthe other experiment examples show that the resistivity is decreased ascompared with in the previous experiment examples.

FIG. 6A and FIG. 6B are each an enlargement of a part of FIG. 5 . Aregion that is the subject of the present invention described in thepresent application is represented with a dashed line (see regions X₁and X₂ in FIG. 6A and FIG. 6B). The border between region X₁ and regionX₂ (these regions will also be collectively referred to as “region X”)is a line at which the resistivity is 0.190 mΩ·cm.

The corners of parallelogram including regions X₁ and X₂ have coordinatevalues of (1.8×10E+20: 0.25 mΩ·cm), (3.6×10E+20: 0.25 mΩ·cm),(3.0×10E+20: 0.15 mΩ·cm) and (6.0×10E+20: 0.15 mΩ·cm). In the regionenclosed by these four points, the mobility is about 70 to about 140cm²/(V·s). It is easy to stably manufacture a high concentration n-typecompound semiconductor by controlling three parameters of theresistivity, the mobility and the electron concentration. A productcorresponding to the conditions represented by region X₁ or region X₂may be manufactured in accordance with the value of the resistivityrequired by the use or the specifications. In FIG. 6A and FIG. 6B, twopoints represented by small squares (squares including “*”) in region X₁correspond to the experiment examples disclosed in Non-patent Document13 mentioned above. In this manner, a high concentration n-type compoundsemiconductor exhibiting desired properties may be manufactured byadjusting the production conditions represented by region X.

FIG. 7 and FIG. 8 are graphs showing various characteristics (electronmobility, temperature dependence, and the like) of the highconcentration n-type GaN shown in Non-patent Document 13. In FIG. 7 ,the vertical axis represents the electron mobility, and the horizontalaxis represents the electron concentration. Hereinafter, the presentinvention including a first invention (invention of the applicationbased on which the present application claims priority) and a secondinvention (addition in the PCT application) disclosed in theabove-mentioned PCT application will be described.

According to embodiment 1, it is indispensable to fulfill the numericalvalue range conditions enclosed by the four points mentioned in (a) to(d) or (a-1) to (d-1).

In each of the above-described embodiments of the present invention, itis preferred that the following structures are provided.

In each of the embodiments, it is preferred that the absorptioncoefficient is 2000 cm⁻¹ or less to light having a wavelength region of405 nm. In each of the embodiments, it is preferred that the absorptioncoefficient is 1000 cm⁻¹ or less to light having a wavelength region of450 nm.

According to the present invention, the term “two-component nitride”refers to a compound of one element among B, Al, Ga and In, andnitrogen. Namely, the “two-component nitride” is a two-component mixedcrystal of BN (boron nitride), AlN (aluminum nitride), GaN (galliumnitride) or InN (indium nitride).

A three-component nitride refers to a compound obtained as a result ofany one of the two-component group 13 elements mentioned above beingpartially replaced with another group 13 element. The three-componentnitride is, for example, a three-component mixed crystal of InGaN(indium gallium nitride), AlGaN (aluminum gallium nitride), or AlInN(aluminum indium nitride). Regarding the three-component compound, it isknown that the composition ratio thereof may be adjusted to adjust thebandgap within the range of the characteristics of the two-componentcompound.

Even a compound containing a trace amount of another group 13 element inaddition to the group 13 element acting as a main component of thecompound semiconductor may also be encompassed in the scope of theabove-described invention. Combinations of elements are arbitrary aslong as the effects of the present invention are not impaired.

Another invention having a different combination of elements is directedto a nitride semiconductor that contains nitrogen and at least one group13 element selected from the group consisting of B, Al, Ga and In, hasan n-type conductivity, and has an electron concentration and aresistivity fulfilling the numerical value conditions enclosed by thefour points mentioned in (a) to (d) or (a-1) to (d-1) in embodiment 1.

In an embodiment of the present invention, a preferred numerical valuerange is, for example, a resistivity of 0.20×10⁻³ Ω·cm or lower and anelectron mobility of 70 to 140 cm²/(V·s).

A more preferable numerical value range is a predetermined range of aresistivity of 0.18×10⁻³ Ω·cm or lower and 0.15×10⁻³ Ω·cm or higher andan electron mobility of 70 to 140 cm²/(V·s) (see region X₁ in FIG. 6Aand FIG. 6B).

Preferably, the nitride semiconductor has a contact resistance of 1×10⁻⁴SIcm² or lower against an n-type ohmic electrode metal.

In an embodiment, the nitride semiconductor contains oxygen as animpurity at 1×10¹⁷ cm⁻³ or higher.

Preferably, the nitride semiconductor has an absorption coefficient of2000 cm⁻¹ or less to light having a wavelength region of 405 nm.Preferably, the nitride semiconductor has an absorption coefficient of1000 cm⁻¹ or less to light having a wavelength region of 450 nm.Preferably, the nitride semiconductor has an RMS value of 5.0 nm or lessobtained by a surface roughness measurement performed by an AFM.

In an embodiment, the at least one group 13 element is Ga.

In an embodiment, the nitride semiconductor contains either one of, orboth of, Si and Ge as donor impurities.

The above-described invention is applicable to a contact structure,comprising the nitride semiconductor for a conductive portion.

The above-described invention is also applicable to a contact structure,comprising the nitride semiconductor for an electrode. Such a contactstructure is usable in a semiconductor device.

Advantageous Effects of Invention

A nitride compound according to the present invention exhibits a lowresistivity of 0.25×10⁻³ Ω·cm or lower even in a high electronconcentration range of about 1.8×10²⁰ cm⁻³ or higher, and also shows anelectron mobility of 70 cm²/(V·s) or higher.

It should be noted that there are cases where a low resistivity of0.19×10⁻³ Ω·cm or lower is almost unnecessary, depending on thespecification, the use or the like of the semiconductor device. In suchcases, the conditions for the manufacturing process (gas, cathode power,electron concentration of the target) may be adjusted in considerationof the productivity to manufacture a compound semiconductor having aresistivity of about 0.20 to about 0.25×10⁻³ Ω·cm, and adapt thecompound semiconductor to a structure portion in which the device isrequired.

According to the present invention, the pulsed sputtering method (PSDmethod) is used to form a sputtered single crystal film with nohigh-temperature process. More preferably, a compound semiconductor filmis formed in a process performed generally at room temperature. There isno limit on the area size of the substrate, and films of various sizesfrom a small size to a large size may be manufactured.

For example, a compound semiconductor film of a rectangular outer shapehaving a length of a side of 2 inches or longer or a compoundsemiconductor film of a circular outer shape having a diameter of 2inches or longer may be formed. Alternatively, a compound semiconductorfilm having an area size that is 30 cm² or larger and having anallowable area within the restriction of the internal space of thesputtering apparatus may be formed.

In this case, a high quality compound semiconductor film is easilyformed with no need of a buffer layer, which is required by theconventional technology.

Now, the properties of the compound semiconductor according to thepresent invention will be described. The resistivity ρ of an n-typenitride semiconductor film is in inverse proportion to the electronmobility μ_(n) and the carrier concentration n (p=(q·n·p_(n))). However,in the present invention, the n-type nitride semiconductor film exhibitsa high electron mobility even at a high electron concentration. Thisindicates that a high quality film having a low electric resistance isformed. Namely, the present invention provides a high quality group 13nitride semiconductor film easily usable for a semiconductor device. Thecompound semiconductor according to the present invention has athreading dislocation density of about 1×10⁶/cm² to about 5×10¹⁰/cm². Anitride compound film preferably having a threading dislocation densityof 10⁵/cm² or lower, namely, in the order of 10³ to 10⁴/cm², may bemanufactured.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a graph showing the relationship between the electronconcentration and the growth rate according to the present invention.

FIG. 2 is a graph showing the relationship between the resistivity andthe growth rate according to the present invention.

FIG. 3 is a graph showing exponential fitting performed on theresistivity and the growth rate according to the present invention.

FIG. 4 is a scatter graph of the electron concentration and theresistivity according to the present invention.

FIG. 5 is a scatter graph of the electron concentration and theresistivity of high concentration n-type nitride semiconductorsaccording to the present invention and the conventional art.

FIG. 6A is a scatter graph of the electron concentration and theresistivity of the high concentration n-type nitride semiconductors(enlarged graph).

FIG. 6B is a scatter graph of the electron concentration and theresistivity of the high concentration n-type nitride semiconductors(graph with additional information).

FIG. 7 is a graph, according to the present invention, showing theexperimental results on the electron mobility of high concentrationn-type GaN as a function of the electron concentration (n(RT)) and alsoshowing the electron mobility that is calculated by expression (1) withthe compensation ratio θ being changed (cited from Non-patent Document13: APL MATERIALS 5, 126102 (2017), “Electron transport properties ofdegenerate n-type GaN prepared by pulsed sputtering”).

FIG. 8(a) is a graph, according to the present invention, showing thetemperature dependence of the electron mobility of Si-doped GaN in thecase where the electron concentration is 3.3×10²⁰ cm⁻³ at roomtemperature; FIG. 8(b) is a graph showing the temperature dependence ofthe electron mobility and the fitting curve of Si-doped samples andGe-doped samples; and FIG. 8(c) is a graph showing the nonparaboliccoefficient α and the compensation ratio θ, obtained by fitting, as afunction of the electron concentration at room temperature (cited fromNon-patent Document 13: APL MATERIALS 5, 126102 (2017), “Electrontransport properties of degenerate n-type GaN prepared by pulsedsputtering”).

FIG. 9A shows a surface AFM image of an Mg-doped GaN thin film.

FIG. 9B is a graph provided to evaluate the activation energy of an Mgacceptor (cited from Proceedings, The Japan Society of Applied Physics,60th Spring Meeting, 30a-G21-10, 15-190).

FIG. 10A(a) shows an optical micrograph, and FIG. 10A(b) shows an IVcurve, both of an AlGaN/GaN hetero-junction FET manufactured by a PSDmethod (cited from the final research report of “Creation offunction-expressing nanosystems by process integration”, which is aresearch field of CREST, Japan Science and Technology Agency).

FIG. 10B(a) shows the relationship between the electron concentrationand the electron mobility of GaN thin films manufactured by a pulsedsputtering deposition method; and FIG. 10B(b) shows the temperaturedependence of the hole concentration of Mg-doped GaN (cited from thefinal research report of “Creation of function-expressing nanosystems byprocess integration”, which is a research field of CREST, Japan Scienceand Technology Agency).

FIG. 11 is a graph showing the relationship between the electronconcentration (N_(e)) and the electron mobility (p_(e)) of an Si-dopedn-type GaN film manufactured by a PSD method.

FIGS. 12(a) and 12(b) provide graphs of SIMS data each showing aprofile, in a depth direction, of the oxygen concentration of a GaN filmhaving an Si concentration of 2×10²⁰ cm⁻³.

FIGS. 13(a) and 13(b) show AFM images of surfaces of the Si-doped GaNfilms shown in FIGS. 12(a) and 12(b) formed by sputtering.

FIGS. 14(a) and 14(b) provide graphs respectively showing the absorptioncoefficient and the refractive index of a GaN film having an Siconcentration (electron concentration) of 2×10²⁰ cm⁻³ measured by anellipsometer.

FIGS. 15(A) and 15(B) provide a schematic view (A) and a schematic viewin a planar direction (B) each showing a crystal structure of GaN.

FIG. 16A is a schematic view showing a structure of a sputteringapparatus usable in the present invention.

FIG. 16B is a schematic side view of a sputter gun used in the presentinvention.

FIG. 17 is a graph showing an example of pulse sequence to be applied toan electrode of the sputtering apparatus at the time of sputteringaccording to the present invention.

FIG. 18 is a schematic vertical cross-sectional view showing an innerstructure of a sputtering apparatus usable in the present invention.

FIG. 19 is a schematic cross-sectional view of a semiconductor deviceaccording to embodiment 1 of the present invention.

FIG. 20 is a schematic cross-sectional view showing a contact structureaccording to embodiment 2 of the present invention.

FIG. 21 is a schematic cross-sectional view showing a contact structureaccording to embodiment 3 of the present invention.

FIG. 22 is a schematic cross-sectional view of a thin film transistor towhich the present invention is applicable.

FIG. 23 is a schematic cross-sectional view of an AlGaN/GaN HEMT towhich the present invention is applicable.

FIG. 24 is a schematic cross-sectional view of an LED device to whichthe present invention is applicable.

FIG. 25 is a schematic cross-sectional view of a surface emitting laserdevice to which the present invention is applicable.

FIG. 26 shows the relationship between the electron concentration andthe resistivity of GaN according to the present invention.

FIG. 27 shows the relationship between the concentrations of donorimpurities and the electron concentrations, of the GaN according to thepresent invention, obtained by a SIMS measurement.

FIGS. 28(a), 28(b), 28(c), and 28(d) provide AFM images of surfaces ofGe-doped GaN samples as examples of surface state of GaN.

FIG. 29 is a schematic cross-sectional view of a vertical power MOSFET.

FIG. 30 is a schematic cross-sectional view of a GaN-based LED.

FIG. 31 is a schematic cross-sectional view of a Schottky diode.

DESCRIPTION OF EMBODIMENTS

Hereinafter, a compound semiconductor manufactured by pulsed sputteringusing a group 13 nitride semiconductor will be described as anembodiment according to the present invention with reference to thedrawings.

A group 13 nitride semiconductor according to an embodiment of thepresent invention is formed as a film by a pulsed sputtering depositionmethod (PSD method).

(Pulse Sputtering Method)

The “pulsed sputtering method (PSD method)” used to manufacture acompound semiconductor of a nitride according to the present invention,and the materials and the manufacturing method used to manufacture thecompound semiconductor, are basic items well known to a person ofordinary skill in the art.

For example, the standard technologies disclosed in the followingpublications are usable to work the present invention with no problem:“Growth of a nitride substrate and a lattice-matched substrate anddevice characteristics” (CMC Publishing Co., Ltd.; first editionpublished on Oct. 30, 2009), “New development of high frequencysemiconductor materials and devices” (CMC Publishing Co., Ltd.; firstedition published on Nov. 13, 2006), “Improvement in performance ofnext-generation power semiconductors and industrial development thereof”(CMC Publishing Co., Ltd.; first edition published on Jun. 10, 2015),Japanese Laid-Open Patent Publication No. 2009-138235 “Pulse sputteringapparatus, and pulsed sputtering method”, Japanese Laid-Open PatentPublication No. 2014-159368 “Gallium nitride sintered body or galliumnitride molded article, and method for producing the same”, and thelike. Patent Documents 2 and 3, and Non-patent Documents 3 and 4, andthe like may also be referred to.

According to the PSD method used in the present invention, crystalgrowth is carried out based on a physical reaction, and therefore, maybe performed at a low temperature. In addition, carbon and hydrogen in afilm formation environment are conspicuously removable. Since thecrystal growth may be performed at a low temperature, generation of athermal stress in the film is suppressed, and also a compound easilyleading to phase separation such as, for example, InGaN, is stablygrown.

Single crystal growth of a compound semiconductor according to thepresent invention is not visually recognizable directly, but theprinciple of action of the crystal growth is generally considered asfollows. FIG. 15 shows a crystal structure of GaN, which is one oftwo-component group 13 compounds. During the film formation of acompound semiconductor according to the present invention, it isconsidered that a polar surface at which Ga atoms of GaN are located toform a hexagonal shape (Ga atom surface) is aligned to a surface of asubstrate acting as an underlying layer, so that a single crystalstructure is formed.

In this step, with the manufacturing method used in the presentinvention, the film formation is allowed to be performed at a relativelylow temperature, instead of at a high temperature exceeding 1000° C.required by the MOCVD method or the like. The temperature range to beused is 700° C. or lower and may include room temperature of 25° C.(room temperature to 700° C.). Although the temperature varies inaccordance with the film formation rate, a preferable temperature rangemay be, for example, 300 to 700° C.

For this reason, it is estimated that a small number of oxygen atomscontained in the film formation atmosphere are present to cover asurface of the film to be formed during the film formation. It isconsidered that as a result of the above, the oxygen atoms act toprevent the bonding of the group 13 element and nitrogen, and therefore,the film formation process advances while main elements to form thedesired compound are kept free.

In addition, it is considered that since the film formation conditionsare the same for the entirety of the underlying layer in a planardirection, a crystal structure that is uniform and has a high level ofcrystallinity entirely is formed.

The GaN compound semiconductor formed as a sputtered film in this manneris considered to gradually grow in an axial direction of the hexagonalshape (thickness direction of the film), so that in a final step, acompound semiconductor film that is uniform in the plane and has atleast a certain area size is manufactured.

It is preferred that the underlying layer to be used is formed of amaterial fulfilling the condition of having a lattice matched with, ormatched in a pseudo manner with, the compound semiconductor to be grown.The film formation process by the PSD method is not performed at a hightemperature exceeding 1000° C. Therefore, the material of the underlyinglayer does not need to be resistant against a high temperature. However,in order to improve the crystallinity, it is preferred that the crystaland the underlying layer fulfill the conditions of being lattice-matchedor pseudo-lattice-matched with each other.

For the above-described reasons, according to the present invention, itis especially preferred that the material of the underlying layer isselected from the four types: SiC, sapphire, GaN, single crystallinesilicon. Sapphire has a heat resistant temperature of 1200° C., andsingle crystalline silicon has a heat resistant temperature of 1100° C.These materials are usable to manufacture semiconductor devices such asAlGaN/GaN HEMTs, full-color LEDs, InGaN-TFTs, sensors and the like.

Alternatively, the material of the underlying layer may be, for example,metal foil or alkali-free glass for FPD having a heat resistanttemperature of 600 to 700° C., or the like, although the formed crystalquality of the compound semiconductor is inferior to the quality in thecase where the above-listed materials are used. In this case, it ispreferred that a buffer layer is formed on a surface of the material ofthe underlying layer for the crystal growth, for the purpose of makingthe underlying layer pseudo-lattice-matched with the compoundsemiconductor.

Regarding the size of the film to be formed according to the presentinvention, a device having a length of a shorter side or a diameter of acircle of 2 inches to 10 inches may be manufactured. The presentinvention is also applicable to a medium-sized device having a diagonalline of a rectangle of 10 to 30 inches and a large device having adiagonal line of a rectangle of 30 inches or longer. The device or thesubstrate acting as the underlying layer may be circular, square,rectangular, or of an asymmetrical shape.

FIG. 16A, FIG. 16B and FIG. 17 show a schematic view of a sputteringapparatus and a pulse sequence usable to manufacture a compoundsemiconductor according to the present invention. A sputtering apparatus1 mainly includes a chamber 11, a substrate electrode 12, a targetelectrode 13, a DC power supply 14, a power supply controller 15, anitrogen supply source 16, a heating device 17, an argon supply source18 and the like.

The chamber 11 is sealable against the outside. The inner pressure ofthe chamber 11 is allowed to be decreased by a vacuum pump or the like(not shown). The substrate electrode 12 is located in the chamber 11,and is capable of holding a heat dissipation sheet 12 a.

A sputter source (or a sputter gun) 13 includes a cylindrical head 13 bon which a sputter material 13 a is placed, and a shaft 13 c connectedwith the cylindrical head 13 b. A power source line 13 d is provided inthe shaft 13 c. An effective size of the head is about 1 inch to about 4inches. The sputter source 13 is provided in the chamber 11 so as toface the substrate electrode 12, and is capable of holding the target 13a.

A distance between a surface of the substrate and the sputter source 13is represented by L_(H). In an embodiment of the present invention,distance L_(H) may be set to about 10 to about 50 cm. Distance L_(H) ismore preferably set to 15 to 40 cm, and is still more preferably set to20 to 30 cm.

The target 13 a is formed of a compound of a group 13 element andnitrogen. A high quality target material with little impurities that iscommonly available at present is used. For example, a high qualitymaterial such as five nine or six nine is needed. The shape and the sizeof the sputter gun may be adjusted when necessary in accordance with thetype of the target to be used. For a batch process, for example, acircular target having a long diameter or a linear (rectangular) targetmay be used. Needless to say, a plurality of sputter guns may be locatedin the chamber. A film of a compound semiconductor according to thepresent invention may be formed either with a target of Ga, Al or Si orwith a target of GaN or AlN.

The DC power supply 14 is electrically connected with the substrateelectrode 12 and the sputter source 13, and is a voltage source thatapplies a DC voltage between the substrate electrode 12 and the sputtersource 13.

The power supply controller 15 is connected with the DC power supply 14,and performs control regarding the timing of the operation of the DCpower supply 14. The power supply controller 15 allows a pulse voltageto be applied between the substrate electrode 12 and the sputter source13.

The nitrogen supply source 16 is connected with the inside of thechamber 11 by, a supply tube or the like, and supplies nitrogen gas intothe chamber 11. The argon supply source 18 supplying argon gas isprovided to generate plasma required for sputtering.

An oxygen supply source that supplies a predetermined amount of oxygenis also provided. The internal pressure is constantly allowed to bemonitored while the film is formed. The content of oxygen in the chamberneeds to be controlled to be kept at about 10 ppm nearly constantlyduring the film formation of the compound semiconductor. Alternatively,the sputtering may be performed while the content of oxygen contained asimpurities in main supply gas is controlled. Still alternatively, theapproximate amount of an oxygen component contained in trace amount inthe other materials may be estimated, so that the amount of the oxygencomponent contained in the nitride semiconductor may be suppressed to bewithin a predetermined level in the entire process.

In order to realize this, it is indispensable that the chamber used forthe pulsed sputtering, the supply system of the process gas and thedischarge system of the process gas (main discharger, rough discharger)prohibit gas leak and invasion of external air, and it is important thatthe pressure is controlled to be highly stable during the filmformation. It is considered to be fundamental to supply a trace amountof oxygen into the chamber intentionally. In order to realize this, thechamber needs to be confirmed to have been cleaned, and the materials tobe used need to have a high purity.

The heating device 17 is secured to, for example, the substrateelectrode 12, so that the temperature around the heat dissipation sheet12 a on the substrate electrode 12 can be adjusted. The representativeexamples of the film formation conditions to be used according to thepresent invention are as follows. FIG. 7 is an example of pulsesequence. The voltage P_(A) of the driving pulse is adjustable. The filmformation rate is generally 0.1 to 4 nm/sec. on average, and morepreferably 0.2 to 2 nm/sec. The film formation rate is 0.025 nm/sec. to0.125 nm/sec. in a high concentration region in which the electronconcentration is 2×10²⁰ cm⁻³.

(a) Driving method: pulsed sputtering method (PSD method)

(b) Duty ratio: 5%

(c) Average power: 100 V

(d) Pulse frequency: 1 kHz

(e) Growth pressure: 2×10⁻³ Torr

(f) Dopant: Si

The film formation by the sputtering was performed in atmospheric gascontaining argon as a main component, and the substrate temperatureduring the film formation was set to the range of 300 to 700° C. In thiscase, doping gas such as SiH₄, GeH₄ or the like is usable as the dopingmaterial, and a target containing Si or Ge atoms is usable, in order toform a high concentration n-type group 13 nitride compoundsemiconductor.

Experiments were made in which oxygen were incorporated at aconcentration of 10 ppm into the atmospheric gas to be used for thesputtering in order to introduce oxygen into the film of the targetcompound semiconductor to be manufactured, and in which oxygen was notincorporated. Physical characteristics of the compound semiconductormanufactured with oxygen and the compound semiconductor manufacturedwith no oxygen were checked in comparison with each other.

FIG. 18 is a schematic vertical cross-sectional view of a continuousfilm formation apparatus 10 of a roll-to-roll system. A plurality offilm formation chambers 5 are provided inside the continuous filmformation device 10. The present invention is applicable to such adevice as long as the substrate film 4 is a metal foil or a very thinfilm-like glass substrate that may be taken into a roll or taken out ofa roll. While the flexible substrate film 4 is transported in ahorizontal direction from a take-out roll 2 to a take-in roll 3, thesputtering may be performed toward the substrate film 4 at a pluralityof locations in the film formation chamber. As a result, a semiconductordevice containing a desired compound semiconductor or the like isprocessed at a high speed. The table in the chamber is usable for, forexample, a diameter of 320 to 600 mm. In a roll-to-roll process, thereis a high possibility that the growth rate changes moment by moment. Insuch a case, an effective growth rate may be assumed for management. Thegrowth rate is considered to be generally lower than in a batch process.

According to the present invention, crystal growth of a compoundsemiconductor is realized on an underlying layer or a substrate havingan area size defined by a shorter side of a rectangle or a diameter ofat least 2 inches. The crystal is manufactured at a low temperature andat a high rate so as to have a certain area size and to be uniform. Inaddition, a novel compound semiconductor is mass-manufactured while themanufacturing cost thereof is suppressed.

FIG. 11 shows the relationship between the electron concentration(N_(e)) and the electron mobility (μ_(e)) of an Si-doped n-type GaN filmmanufactured by the PSD method by the present inventors. The electronconcentration and the electron mobility were measured by a roomtemperature Hall effect measurement. In the experiment example providingdata plotted in this figure, about 2×10²⁰ cm⁻³ or the vicinity thereofis the upper limit. The electron concentration (N_(e)) is considered tobe substantially equal to the Si donor concentration. The film formationby the sputtering was performed in atmospheric gas containing argon gasas a main component, and the substrate temperature during the filmformation was in the range of 300 to 700° C.

Oxygen was incorporated at a concentration of 10 ppm into theatmospheric gas to be used for the sputtering for the purpose ofintroducing oxygen into this film, so that a crystal film exhibitingsingle crystallinity was formed. An n-type ohmic electrode metal stackstructure (Ti (20 nm)/AI (60 nm)/Ti (20 nm)/Au (50 nm)) was formed on asurface of the resultant GaN thin film, and was annealed in nitrogen at700° C. The contact resistance of samples formed in this manner wasevaluated by a TLM method and was found to be 8.5×10⁻⁵ Ω·cm².

In this figure, the circles show the actually measured values, and thecurve shows the fitting result based on the Caughey-Thomas-typeempirical formula (formula 1 below; see Non-patent Document 4), which isused to describe the mobility in a low electric field. In the formulabelow, N_(D) is the donor concentration. Since the electronconcentration (N_(e)) is considered to be substantially equal to the Sidonor concentration as described above, the fitting is performed with anassumption that N_(D)=N_(e).μ=μ_(min)+[μ_(max)−μ_(min)]/[1+(N _(D) /N _(R))^(γ)]  (1)

From the above-shown fitting result,

μ_(max)=1034 cm²/(V·s),

μ_(min)=125 cm²/(V·s)

were found. These values are comparable to the highest value of themobility of the n-type GaN thin film formed by the MOCVD methodconventionally reported (see, for example, Non-patent Document 5). Ascan be seen, it has been confirmed that the carrier scattering issufficiently suppressed in the film of the compound semiconductormanufactured according to the present invention.

With the MOCVD method of the conventional technology, it is consideredto be difficult to form a GaN thin film exhibiting such a high electronmobility when the donor concentration is generally 5×10¹⁹ cm⁻³ orhigher. According to the present invention, as shown in FIG. 11 , theSi-doped n-type GaN film manufactured by the PSD method exhibits thevalues matching the Caughey-Thomas-type empirical formula (Non-patentDocument 4) even at the donor concentration of at least 2×10²⁰ cm⁻³.

Namely, it has been found out that an n-type GaN film according to thisexample manufactured by the PSD method is a very high quality filmexhibiting an electron mobility of 46 cm²/(V·s) or higher even at anelectron concentration of 5×10¹⁹ cm⁻³ or higher. Preferably, a filmexhibiting an electron mobility of 50 cm²/(V·s) or higher is usable. Theresistivity ρ of an n-type nitride semiconductor film is in inverseproportion to the electron mobility μ_(n) and the carrier concentrationn. Therefore, the n-type nitride semiconductor film exhibits a highelectron mobility even at a high electron concentration. This indicatesthat a high quality film having a low resistance is formed.

The samples shown in FIG. 11 are all Si-doped. The impurity to beincorporated as a donor is not limited to Si and may be Ge or the like.

When the donor concentration of the nitride semiconductor film isincreased in order to realize a high electron concentration, thetransparency of the film to visible light is decreased. This causes aconcern that an inconvenience may occur in the case where the nitridesemiconductor film according to the present invention is used for atransparent electrode or the like.

Under such circumstances, according to the present invention, thedecrease in the transparency caused by the increase in the electronconcentration of the film of the compound semiconductor is compensatedfor as follows. The nitrogen site is replaced, so that oxygen, which isa dopant acting as a donor, is incorporated as an impurity to expand thebandgap of the film.

The bandgap of an oxygen-doped film depends on the amount of doping. Forexample, in the case of GaN, the bandgap at room temperature may bevaried in the range of 3.4 eV to 4.9 eV (value of the bandgap of galliumoxide). In the case of, for example, GaN, when oxygen is incorporated asan impurity at 1×10¹⁷ cm⁻³ or higher into the film, the bandgap at roomtemperature is generally about 3.4 to about 3.6 eV.

Such an effect of oxygen, for example, allows the nitride semiconductorfilm according to this example to have an absorption coefficient of 2000cm⁻¹ or less to light having a wavelength region of 405 nm or to have anabsorption coefficient of 1000 cm⁻¹ or less to light having a wavelengthregion of 450 nm. In this manner, the nitride semiconductor filmaccording to this example is usable for a transparent electrode with noinconvenience.

FIG. 12 provides graphs each showing an oxygen concentration of the GaNfilm according to this embodiment manufactured by the PSD method. FIG.12(b) shows SIMS data representing a profile, in a depth direction, ofthe oxygen concentration of a GaN film having an Si concentration of2×10²⁰ cm⁻³, among the samples shown in FIG. 11 . It is understood thatthe oxygen is contained at a concentration of about 1 to 3×10¹⁸ cm⁻³.This film exhibits an electron mobility of 110 cm²/(V·s).

The RMS value of an AFM image representing the surface roughness of thisfilm was 3.97 nm as seen from FIG. 13(b). The present inventorsperformed an AFM measurement on the film samples formed by the presentinventors at various electron concentrations and containing oxygen at anelectron concentration of 5×10¹⁹ cm⁻³ or higher. All the samples had anRMS value of 5.0 nm or less.

In the meantime, crystal growth was performed under substantially thesame conditions but with no incorporation of 10 ppm oxygen into theatmospheric gas. The results were as follows. As shown in the profile inFIG. 12(a), the oxygen concentration was about 1×10¹⁶ cm⁻³, and themobility at this point was 45 cm²/(V·s). As seen from FIG. 13(a), theRMS value representing the surface roughness of this thin film was 14.1nm.

Now, the two conditions, namely, the condition of incorporating oxygenand the condition of not incorporating oxygen, will be discussed. In thecase with oxygen, it is considered that oxygen atoms in the atmospherecovering a surface of the film that is being formed cause the stress tobe alleviated and the migration of the atoms at the surface to promote.It is considered that such suppression of the surface roughness avoidsintroduction of point defects and thus improves the mobility. At a hightemperature used by the MOCVD method or the like of the conventionaltechnology, oxygen evaporates from the surface. Therefore, it isconsidered to be difficult to provide the effect of improving thequality realized by the low-temperature growth performed by the PSDmethod.

By contrast, in the case with no oxygen, it is considered that theabove-described action is not easily provided and thus the crystal ofthe film formed by the PSD method is likely to include defects.

FIG. 14 provides graphs showing the absorption coefficient (FIG. 14(a))and the refractive index (FIG. 14(b)) of a GaN film having an Siconcentration (electron concentration) of 2×10²⁰ cm⁻³ measured by anellipsometer. This film exhibits an electron mobility of 115 cm²/V·s.This film has an absorption coefficient of 844 cm⁻¹ at a wavelength of450 nm, which is used for a blue LED as a standard wavelength, and hasan absorption coefficient of 1860 cm⁻¹ at a wavelength of 405 nm, whichis used for a blue-violet laser as a standard wavelength.

As can be seen, the oxygen doping allows the film to have an absorptioncoefficient of 2000 cm⁻¹ or less to light having a wavelength region of405 nm or to have an absorption coefficient of 1000 cm⁻¹ or less tolight having a wavelength region of 450 nm. As a result, the obtainedcompound semiconductor is usable as a transparent material.

Hereinafter, various forms of electronic device to which a compoundsemiconductor according to the present invention is applicable.

Embodiment 1

FIG. 19 is a schematic cross-sectional view of a compound semiconductordevice 20 including a group 13 nitride semiconductor formed on asubstrate. Reference sign 21 represents the substrate (sapphire), andreference sign 22 represents GaN.

Embodiment 2

FIG. 20 is a schematic cross-sectional view of a contact structureformed using a compound semiconductor according to the presentinvention. Reference sign 31 represents a GaN substrate, reference sign32 represents GaN (film of a compound semiconductor formed by the PSDmethod), reference sign 34 represents an insulating film, reference sign33 represents a wiring electrode connectable with an external device,and reference sign 35 represents a contact hole.

Embodiment 3

FIG. 21 is a schematic cross-sectional view of a contact structure 40formed using a group 13 nitride compound semiconductor according to thepresent invention. In FIG. 21 , reference sign 41 represents an n-typeGaN contact layer, reference sign 42 represents a Ti layer, referencesign 43 represents an Al layer, reference sign 44 represents an Nilayer, and reference sign 45 represents an Au layer. In this example, acomposite metal electrode is used. After the film formation, heattreatment is performed at about 900° C.

Application Examples

FIG. 22 is a schematic cross-sectional view of a thin film transistor towhich the present invention is applicable. A high concentration n-typeGaN layer is applicable as a contact layer of an electrode of the thinfilm transistor.

In the figure, reference sign 51 represents a substrate formed ofalkali-free glass or the like, reference sign 52 represents aninterlayer insulating layer, reference sign 53S represents a source-sidecontact layer (high concentration n⁺ GaN layer), reference sign 54Srepresents a source region, reference sign 55 represents an activelayer, reference sign 54D represents a drain region, reference sign 53Drepresents a drain-side contact layer (high concentration n⁺ GaN layer),reference sign 56 represents a gate oxide film, reference sign 57represents a source electrode, reference sign 58 represents a gateelectrode, and reference sign 59 represents a drain electrode. Thesource region 54S and the drain region 54D are each formed such that theconcentration of the impurity is gradually changed between thecorresponding contact layer and the active layer.

FIG. 23 is a schematic cross-sectional view of a HEMT to which thepresent invention is applicable. A high concentration n-type GaN layeraccording to the present invention is applicable as contact layerslocated below, and in contact with, source and drain electrodes of theAlGaN/GaN-HEMT. In the figure, reference sign 61 represents a substrateformed of GaN, sapphire, SiC, Si or the like, reference sign 62represents a buffer layer formed of GaN, AlN or the like, reference sign63 represents a GaN undoped layer, reference sign 64 represents an AlGaNbarrier layer, and reference sign 65 represents a contact layer formedusing a high concentration n-type GaN layer. A source electrode 66, agate electrode 67 and a drain electrode 68 are provided in a top part ofthe HEMT.

In the thin film transistor (FIG. 22 ) and the HEMT (FIG. 23 ) describedabove, the high concentration n-type GaN layer is applicable as thecontact layer. The contact resistance of such a contact layer against anelectrode in a circuit element in which an operating current flows (thecircuit element is each of source and drain in the thin film transistorand the HEMT) is significantly decreased. This significantly contributesto the improvement in performance of the electronic device.

FIG. 24 is a schematic cross-sectional view of an LED device as anexample of GaN-based semiconductor device to which the present inventionis applicable.

As shown in this figure, a plurality of compound semiconductor layersare sequentially stacked from the side of a substrate 71 formed of GaN,sapphire, SiC or Si. A buffer layer 72, an n-type GaN layer 73, aGaInN/GaN MQW light emitting layer 74, a p-type GaN layer 75, a tunneljunction 76 including a p-type InGaN layer 76 a and a high concentrationn-type GaN layer 76 b, an n-type GaN layer 77, a contact layer 78 formedof a high concentration n-type GaN layer, and electrodes 79A and 79B areprovided.

FIG. 25 is a schematic cross-sectional view of an InGaN/GaN VCSEL(surface emitting laser) structure to which the present invention isapplicable. In such a vertical cavity surface emitting laser (VCSEL), aresonator is formed to be perpendicular to a surface of a semiconductorsubstrate. Therefore, laser light is output perpendicularly to thesubstrate surface.

In the figure, reference sign 81 represents a GaN substrate, referencesign 82D represents an inner multi-layer reflection mirror, referencesign 83 represents an n-type GaN layer, reference sign 84 represents anMQW active layer formed of GaInN/GaN, reference sign 85 represents ap-type AlGaN layer, reference sign 86 a represents a p-type InGaN layer,and reference sign 86 b represents a high concentration n-type GaNlayer. 86 a and 86 b form a tunnel junction 86. Reference sign 87represents an n-type GaN layer, reference sign 88 represents a highconcentration n-type GaN layer (contact layer), reference sign 89A and89B represent electrodes, and reference sign 82U represents an uppermulti-layer reflection mirror.

As described above, the compound semiconductor according to the presentinvention is usable for, for example, regions of a light emitting deviceor an electronic device in which a large amount of electric currentflows, a contact portion of a semiconductor device, or an electrodestructure such as a transparent electrode or the like. The compoundsemiconductor according to the present invention is preferably usablefor a wire or the like of an electronic device drivable at a very lowvoltage. The compound semiconductor according to the present inventionis adaptable to the specifications of large electric current and largeelectric power, which are not easily dealt with by the conventionaltechnology.

The compound semiconductor according to the present invention exhibits ahigh electron mobility and thus has a low resistance, and therefore isconsidered to contribute to improvement in the operation speed ofdevices.

So far, a compound semiconductor according to the first invention,namely, a two-, three- or four-component compound semiconductor thatcontains nitrogen and one element selected from the group consisting ofB, Al, Ga and In, which are group 13 elements, contains oxygen as animpurity at 1×10¹⁷ cm⁻³ or higher, has an electron concentration of5×10¹⁹ cm⁻³ or higher, has n-type conductivity and exhibits an electronmobility of 46 cm²/(V·s) or higher has been described.

Hereinafter, a nitride semiconductor according to a second inventionmade by the present inventors will be described.

The nitride semiconductor has a conspicuous feature of exhibiting alower resistivity (namely, exhibiting a higher mobility) than aconventional semiconductor although being in the form of a crystal dopedwith a donor at a high concentration.

Specifically, the nitride semiconductor contains nitrogen and at leastone group 13 element selected from the group consisting of B, Al, Ga andIn, has n-type conductivity, exhibits an electron concentration of1×10²⁰ cm⁻³ or higher, and exhibits a resistivity of 0.3×10⁻³ Ω·cm orlower. Preferably, the at least one group 13 element is Ga, and eitherone of, or both of, Si and Ge are contained as donor impurities.

Conventionally, a nitride semiconductor doped with Ge grown by the MBEmethod at a high concentration and exhibiting a relatively lowresistivity is known. As compared with such a nitride semiconductor, thenitride semiconductor according to the present invention realizes alower resistivity in a region having a higher electron concentration.

Such a nitride semiconductor exhibiting a low resistivity (exhibiting ahigh mobility) although being in the form of a crystal doped with donorsat a high concentration is expected to be used for various uses, forexample, to decrease the parasitic resistance of an electronic devicesuch as an HEMT or the like, to provide a material replacing atransparent conductive film of ITO or the like, and to realize cascadeconnection of LED modules.

FIG. 26 shows the relationship between the electron concentration (cm⁻³)and the resistivity (mΩ·cm) of GaN according to the present invention.In the figure, star marks represent the GaN according to the presentinvention. Among the star marks, white star marks represent Si-dopedGaN, and gray star marks represent Ge-doped GaN. The figure also shows,for comparison, data of GaN obtained by the MOCVD method (diamond-shapedmarks) and the MBE method (circular marks) reported so far, and alsoshows the relationship between the electron concentration and theresistivity obtained by a theoretical calculation. In the figure, θrepresents the compensation ratio of the concentration of ionizedimpurities (ratio of the acceptor concentration N_(A) and the donorconcentration N_(D); N_(A)/N_(D)). (Note: FIG. 26 is the same as FIG. 4of Non-patent Document 13 mentioned above except for one experimentexample having the lowest resistivity shown in a bottom part of thegraph.)

The GaN crystal conventionally reported exhibits a tendency that theresistivity is decreased as the electron concentration is increasedregardless of whether the crystal is obtained by the MBE method or theMOCVD method. However, the resistivity is increased when the electronconcentration is above a certain level.

For example, in the case of GaN obtained by the MOCVD method, Si-dopedGaN shows an increase in the resistivity when the electron concentrationexceeds about 5×10¹⁹ cm⁻³, and Ge-doped GaN shows an increase in theresistivity when the electron concentration exceeds about 1×10²⁰ cm⁻³.In the case of GaN obtained by the MBE method, Si-doped GaN shows anincrease in the resistivity when the electron concentration exceedsabout 1.5×10²⁰ cm⁻³, and Ge-doped GaN shows an increase in theresistivity from when the electron concentration exceeds about 5×10²⁰cm⁻³.

By contrast, in the case of GaN according to the present invention,neither Si-doped GaN (white marks) nor Ge-doped GaN (gray marks) showsany such increase in the resistivity even when the electronconcentration is 5×10²⁰ cm⁻³.

In addition, in the case of the conventional GaN, even Ge-doped GaN,obtained by the MBE method and exhibiting the lowest resistivity in aregion of a high electron concentration, exhibits a resistivity ofmerely 0.4 mΩ·cm (0.4×10⁻³ Ω·cm) at the minimum at an electronconcentration of about 5×10²⁰ cm⁻³. By contrast, the GaN according tothe present invention exhibits a resistivity of 0.2 mΩ·cm (0.2×10⁻³Ω·cm) at nearly the same electron concentration.

As is clear from the results shown in this figure, unlike theconventional GaN, the GaN according to the present invention has afeature of exhibiting a conspicuously low resistivity of 0.3×10⁻³ Ω·cmor lower especially when the electron concentration is 1×10²⁰ cm⁻³ orhigher, and this feature is not lost even when the electronconcentration is 2×10²⁰ cm⁻³ or higher. As shown in the table below,this tendency has been experimentally confirmed in the range ofresistivity down to 0.16×10⁻³ Ω·cm. The theoretical value of the lowestlimit of the resistance value caused by scattering of ionized impuritiesis 0.04×10⁻³ Ω·cm, but is varied to, for example, 0.2×10⁻³ Ω·cm,0.15×10⁻³ Ω·cm, 0.1×10⁻³ Ω·cm or the like depending on the filmformation conditions or the like. As a result of the fitting shown inFIG. 3 , the estimated value of 0.083×10⁻³ Ω·cm was obtained.

FIG. 27 shows the relationship between the concentration of the donorimpurities and the electron concentrations, of the GaN according to thepresent invention, obtained by a SIMS measurement. It is understood fromthese results that the activation ratio of the donors is about 1 in theGaN according to the present invention obtained by the PSD method.Namely, it is understood that for the GaN according to the presentinvention, the electron concentration is controllable by merelycontrolling the doping concentration of the donor impurity.

The various characteristics (electron concentration, electron mobility,resistivity, and surface roughness) of the GaN according to the presentinvention are shown in Table 1 (Si-doped GaN) and Table 2 (Ge-dopedGaN). Table 3 (Si-doped GaN) shows the relationship between the growthrate and various characteristics (electron concentration, electronmobility, resistivity, surface roughness) of Si-doped GaN in a highconcentration region according to the present invention.

TABLE 1 Si-doped GaN Electron Electron Surface roughness concentrationmobility Resistivity RMS value (cm⁻³) (cm²V⁻¹s⁻¹) (mΩ · cm) (nm)1.12E+19 211 2.64 0.85 2.16E+19 159 1.82 0.95 3.02E+19 154 1.34 0.944.75E+19 150 0.876 0.70 8.09E+19 136 0.567 0.90 9.36E+19 128 0.521 0.881.44E+20 126 0.344 0.65 1.47E+20 126 0.337 0.75 1.66E+20 115 0.327 0.551.93E+20 106 0.305 0.88 1.99E+20 110 0.285 0.95 2.03E+20 110 0.279 0.682.95E+20 108 0.196 0.76 3.30E+20 107 0.177 0.81 3.84E+20 99.6 0.165 0.883.95E+20 100 0.160 0.91

TABLE 2 Ge-doped AlGaN Electron Electron Surface roughness concentrationmobility Resistivity RMS value (cm⁻³) (cm²V⁻¹s⁻¹) (mΩ · cm) (nm)1.24E+19 153 3.29 0.91 2.03E+20 96.8 0.280 0.77 2.87E+20 82.1 0.265 0.623.04E+20 79.0 0.260 0.62 3.24E+20 74.6 0.258 0.54 3.28E+20 77.4 0.2460.46 3.36E+20 73.8 0.252 0.46 3.39E+20 70.2 0.262 0.31 3.54E+20 72.20.244 0.34 3.99E+20 73 0.214 0.35 4.11E+20 70.4 0.216 0.65 4.35E+20 70.90.202 0.65 4.49E+20 66.2 0.210 0.55 4.70E+20 66.2 0.200 0.55 5.15E+2060.1 0.202 0.86 5.25E+20 57.8 0.207 0.86 5.49E+20 41.3 0.275 0.86

TABLE 3 Si-doped GaN Electron Electron Surface Growth rate concentrationmobility Resistivity roughness (nm/h) (cm⁻³) (cm²V⁻¹s⁻¹) (mΩ · cm) (nm)200 3.95E+20 100 0.158 0.78 230 3.30E+20 105 0.180 0.66 280 2.86E+20 1190.183 0.85 364 2.08E+20 110 0.273 0.68

The GaN layers shown in Table 1 to Table 3 were all obtained in nearlythe same conditions as the crystal growth conditions by the PSD methoddescribed above. The materials and the like each having the followingpurity were used. The electron concentration was changed by changing thepower applied to the cathode from 20 to 150 W.

Substrate temperature during the growth: 600 to 700° C.

Sputtering target (Si): single crystal having a purity of 99.999%

Sputtering target (Ge): single crystal having a purity of 99.99%

Ga: Purity: 99.99999%

Nitrogen gas: purity: 99.9999%

The present inventors note that the vacuum level of the film formationenvironment and the quality of the vacuum state are important forgrowing a high quality crystal, and appropriately adjusted theconditions of pulsed sputtering (pulse voltage, pulse width, duty ratio,etc.) in order to obtain a crystal of a desired film quality. It is oneof advantages of the PSD method that such fine adjustments may be madequickly.

The measurement conditions and the like for the above-mentioned variousproperties are as follows.

The electron concentration and the electron mobility were measured byuse of a Hall measurement device (ResiTest8400, Toyo Corporation) whilethe applied current was varied in the range of 1 mA to 10 mA and theapplied magnetic field was varied in the range of 0.1 to 0.5 T (tesla)in accordance with the resistivity of the sample. The temperature forthe measurement was room temperature.

The surface roughness was measured by use of an AFM device (JSPM4200produced by JEOL Ltd.).

FIG. 28 shows AFM images of surfaces of the Ge-doped GaN samples asexamples of surface state of the above-described GaN. These samples allhave an RMS value less than 1 nm. In general, a surface having an RMSvalue, obtained by a surface roughness measurement by an AFM device, of5.0 nm or less may be evaluated to be sufficiently flat. Inconsideration of this, it is understood that the nitride semiconductoraccording to the present invention has a highly flat surface.

Nitride semiconductor crystals having the Ga site of GaN be partiallyreplaced with Al or In (AlGaN or InGaN) were also manufactured, andvarious properties thereof were examined. The results are shown in Table4 and Table 5. In these samples, the concentration of Al or In was 1%.The purity and the like of each of the materials used for the crystalgrowth are as follows.

Substrate temperature during the growth: 600 to 700° C.

Sputtering target (Si): single crystal having a purity of 99.999%

Sputtering target (Ge): single crystal having a purity of 99.99%

Ga: Purity: 99.99999%

Al: Purity: 99.999%

In: Purity: 99.999%

Nitrogen gas: purity: 99.9999%

TABLE 4 Ge-doped AlGaN Electron Electron concentration mobilityResistivity (cm⁻³) (cm²V⁻¹s⁻¹) (mΩ · cm) 4.76E+20 61.7 0.213

TABLE 5 Si-doped InGaN Electron Electron concentration mobilityResistivity (cm⁻³) (cm²V⁻¹s⁻¹) (mΩ · cm) 2.32E+20 98.4 0.273

The contact resistance of each of the nitride semiconductors shown inTable 1 to Table 5 was measured. It has been confirmed that all thesamples have a contact resistance of 1×10⁻⁴ Ω·cm² or lower against ann-type ohmic electrode metal. Such a value is sufficiently low. Acontact structure including any of the above-described nitridesemiconductors for a conductive portion is expected to be used invarious uses, for example, to decrease the parasitic resistance of anelectronic device such as a HEMT or the like, to provide a materialreplacing a transparent conductive film of ITO or the like, and torealize cascade connection of an LED module.

The contact resistance was measured by use of a TLM (Transmission LineModel) measurement apparatus (semiconductor parameter analyzer Agilent4155C) on a TLM pattern including Ti/Al/Ti/Au electrode structures (100μm×100 μm) located at an inter-electrode distance of 2 μm to 100 μm.

As described above, the nitrogen site of the nitride semiconductor maybe replaced, so that oxygen, which is a dopant acting as a donor, isincorporated as an impurity to expand the bandgap of the film. In thismanner, the decrease in the transparency caused by the increase in theelectron concentration of the film of the nitride semiconductor iscompensated for.

For this purpose, for example, oxygen as an impurity is incorporated at1×10¹⁷ cm⁻³ or higher into the above-described nitride semiconductor.Such incorporation of oxygen as an impurity allows the nitridesemiconductor to have an absorption coefficient of 2000 cm⁻¹ or less tolight having a wavelength region of 405 nm or to have an absorptioncoefficient of 1000 cm⁻¹ or less to light having a wavelength region of450 nm.

The above-described nitride semiconductor according to the presentinvention is formed by the PSD method. The present inventors considerthat the above-described characteristics are obtained for the followingreason: with other crystal growth methods, the crystal growth is carriedout in a thermal equilibrium state, whereas with the PSD method, thecrystal growth is carried out in a thermal non-equilibrium state.

A nitride semiconductor such as GaN or the like doped with a donor at ahigh concentration is thermodynamically unstable, and therefore, ispartially decomposed even while the crystal growth is being carried out.Namely, the growth and the decomposition of the crystal occur at thesame time. Therefore, the donor impurity once incorporated into thecrystal is pushed out at the time of decomposition. When it is attemptedto dope the nitride semiconductor with donor impurities at a highconcentration, this phenomenon that the donor impurities are pushed outreaches to an unignorable level, and as a result, the crystallinityitself is decreased. Namely, in the case where the nitride semiconductoris doped with the donor impurity at a high concentration, the decreasein the crystallinity is unavoidable under the crystal growth conditionsclose to the thermal equilibrium state.

By contrast, with the PSD method, the crystal growth is carried out in athermal non-equilibrium state. Therefore, the donor impurity is noteasily pushed out, and thus the crystallinity is not easily decreased.

In general, the Ge donor impurities tend to be more easily incorporatedinto the nitride semiconductor crystal at a high concentration than theSi donor impurities. One conceivable reason for this is the following.Since the radius of the Ge ion is close to the radius of Ga ion, the Geion easily replaces the Ga ion site. As a result, the accumulation ofstress in the nitride semiconductor film is alleviated, and thus thesurface of the film tends to be flat.

As described above, the nitride semiconductor according to the presentinvention realizes a lower resistivity in a region of a higher electronconcentration than the conventional nitride semiconductor.

There are the following documents that disclose inventions relating to anitride semiconductor device having a low on-resistance.

Japanese Laid-Open Patent Publication No. 2016-115931 (Patent Document4) discloses an invention relating to a nitride semiconductor devicehaving a low on-resistance. Paragraph 0049 describes that “as describedabove, the source-side nitride semiconductor regrowth layer 205 a andthe drain-side nitride semiconductor regrowth layer 206 a each maycontain n-type impurities at a high concentration. However, as shown inFIG. 4 , when the impurity is silicon (Si), even if an impurity amountto be supplied during the growth of a nitride semiconductor layer isincreased, the carrier concentration of the impurity in the nitridesemiconductor layer to be formed is not increased. That is, the impuritycarrier concentration has a certain upper limit. On the other hand, whengermanium (Ge) is used as the impurity, a higher carrier concentrationthan that of silicon can be realized”.

Paragraph 0095 describes that “in order to investigate thecharacteristics of the composite electrode of the nitride semiconductordevice 200 thus formed, the sheet resistance of the nitridesemiconductor regrown layer itself and the contact resistance thereofwith the 2DEG were measured by a transmission line measurement (TLM)method. FIG. 7 shows the relationship between the sheet resistance ofthe nitride semiconductor regrown layer itself and the supply amount ofGe. It was found that when the flow rate ratio of TEGe to TMG isincreased to 0.09 or more with an increase in supply amount of TEGe, anitride semiconductor regrown layer having a lowered sheet resistance ofapproximately 1.5×10⁻⁶ Ω·cm can be obtained. It was found that when anitride semiconductor regrown layer formed under the conditionsdescribed above is used, the nitride semiconductor device 200 has acontact resistance of 1 to 5×10⁻⁶ Ω·cm, and a preferable contact withthe 2DEG can be obtained”.

Patent Document 4 and the corresponding United States Patent ApplicationPublication US2016/0172473 (Patent Document 5) filed claiming thebenefit of priority to Patent Document 4 were compared against eachother regarding the above description. As a result, it has been foundout that the name and the unit of the vertical axis of FIG. 7 arevariously changed. It is presumed that Patent Document 4 includes sometypographical error.

A technological document written by the inventors of Patent Document 4(IEDM14: Non-patent Document 9), pp. 275-278 (“Extremely lowon-resistance Enhancement-mode GaN-based HFET using Ge-doped regrowthtechnique”) will be referred to. This document discloses a Ge-dopednitride semiconductor regrowth layer exhibiting a low on-resistance.FIG. 3 is exactly the same as FIG. 7 of Patent Document 4.

The vertical axis is labeled as “Specific contact resistance (Ω·cm²)”.Regarding FIG. 3 , there is a description “the measured specific contactresistance as a function of TEGe supply is shown in FIG. 3 , whereextremely low specific contact resistance of 1.5×10⁻⁶ Ω·cm² wasachieved”. For this reason, it is considered that the vertical axis ofFIG. 7 of Patent Document 4 should be “contact resistance” and the unitshould be “Ω·cm²”.

If, as shown in FIG. 7 of Patent Document 4, the resistivity is about1.5×10⁻⁶ Ω·cm and the Ge concentration (electron concentration) is1×10²⁰ cm⁻³, the electron mobility is about 42,000 cm²/(V·s). This valueis far from the normal value known as the electron mobility of GaNcrystal (about 1,200 cm²/(V·s)). Based on this also, it is obvious thatthe above-described portion includes typographical errors.

As described above, Patent Document 4 is considered to disclose a“nitride semiconductor regrowth layer having a lowered contactresistance of approximately 1.5×10⁻⁶ Ω·cm²”.

The above-described nitride semiconductor according to the presentinvention has a feature of exhibiting a low resistivity (exhibiting ahigh mobility) although being in the form of a crystal doped with donorsat a high concentration, and utilizing such a feature, is expected to beused for various uses, for example, to decrease the parasitic resistanceof an electronic device such as an HEMT or the like, to provide amaterial replacing a transparent conductive film of ITO or the like, andto realize cascade connection of an LED module. For example, the nitridesemiconductor according to the present invention may be applied asfollows.

[Application to a Vertical Power MOSFET]

FIG. 29 is a schematic cross-sectional view of a vertical power MOSFET.This vertical power MOSFET 100 includes an n⁺-GaN layer 105 of a nitridesemiconductor, according to the present invention, formed on a stackstructure including an n⁺-GaN layer 102, an n⁻-GaN layer 103 and a p-GaNlayer 104. The n⁺-GaN layer 105 according to the present invention maybe patterned as follows. After being deposited on the entire surface,the n⁺-GaN layer is patterned by lithography. Alternatively, a selectivegrowth technology may be used, according to which, a crystal surface ofgallium nitride is exposed to only a part of a surface of the sample,and the n⁺-GaN layer is epitaxially grown selectively on the exposedpart. Reference sign 106 represents an insulating film, reference sign101 represents a drain, reference sign 107 represents a source, andreference sign 108 represents a gate.

[Application to an LED]

FIG. 30 is a schematic cross-sectional view of a GaN-based LED. The LED200 includes an n-type nitride semiconductor layer 202, an active layer203 including a quantum well layer, a p-type nitride semiconductor layer204, and an n⁺-GaN layer 205 according to the present inventionsequentially stacked on a substrate 201 formed of a nitridesemiconductor.

A cathode electrode 206 is formed on a region of the n-type nitridesemiconductor layer 202 that is exposed as a result of the n⁺-GaN layer205, the p-type nitride semiconductor layer 204 and the active layer 203being partially removed. An anode electrode 207 is formed above thep-type nitride semiconductor layer 204 with the n⁺-GaN layer 205 beinglocated therebetween. The n⁺-GaN layer 205 according to the presentinvention is conductive with the p-type nitride semiconductor layer 204via a tunnel junction.

[Application to a Schottky Diode]

FIG. 31 is a schematic cross-sectional view of a Schottky diode. In thisSchottky diode 300, an n⁺-GaN substrate 301 has an n⁺-GaN layer 306according to the present invention formed on a rear surface thereof. Ann⁻-GaN layer 302 is formed on a front surface of the n⁺-GaN substrate301. An ohmic electrode 303 is formed on the n⁺-GaN layer 306 side, anda Schottky electrode 304 is formed on the n⁻-GaN layer 302 side. In thefigure, reference sign 305 represents an insulating film.

The nitride semiconductor according to the present invention exhibitinga low resistivity (exhibiting a high mobility) although being in theform of a crystal doped with a donor at a high concentration is usablefor an n⁺-GaN layer of, for example, an IGBT (Insulated Gate BipolarTransistor) in addition to the above-described devices.

As described above, the compound semiconductor according to the secondinvention made by the present inventors may be summarized as follows.

The second invention is directed to a nitride semiconductor havingn-type conductivity and containing nitrogen and at least one group 13element selected from the group consisting of B, Al, Ga and In, in whichthe nitride semiconductor has an electron concentration of 1×10²⁰ cm⁻³or higher and exhibits a resistivity of 0.3×10⁻³ Ω·cm or lower.

Preferably, the electron concentration is 2×10²⁰ cm⁻³ or higher.

Preferably, the nitride semiconductor has a contact resistance of 1×10⁻⁴Ω·cm² or lower against an n-type ohmic electrode metal.

In an embodiment, the nitride semiconductor contains oxygen as animpurity at 1×10¹⁷ cm⁻³ or higher.

Preferably, the nitride semiconductor has an absorption coefficient of2000 cm⁻¹ or less to light having a wavelength region of 405 nm.

Preferably, the nitride semiconductor has an absorption coefficient of1000 cm⁻¹ or less to light having a wavelength region of 450 nm.

Preferably, the nitride semiconductor has an RMS value of 5.0 nm or lessobtained by a surface roughness measurement performed by an AFM.

In an embodiment, the at least one group 13 element is Ga.

In an embodiment, the nitride semiconductor contains either one of, orboth of, Si and Ge as donor impurities.

The lower limit of the resistivity is, for example, 0.2×10⁻³ Ω·cm,0.15×10⁻³ Ω·cm, or 0.1×10⁻³ Ω·cm.

The relationship between the electron concentration and the resistivityof the nitride semiconductor fulfills a numerical value range enclosedby four points at which (a) the electron concentration is 1×10²⁰ cm⁻³and the resistivity is 0.3×10⁻³ Ω·cm, (b) the electron concentration is3×10²⁰ cm⁻³ and the resistivity is 0.3×10⁻³ Ω·cm, (c) the electronconcentration is 4×10²⁰ cm⁻³ and the resistivity is 0.15×10⁻³ Ω·cm, and(d) the electron concentration is 9×10²⁰ cm⁻³ and the resistivity is0.15×10⁻³ Ω·cm.

The above-described invention is applicable to a contact structure,comprising the nitride semiconductor for a conductive portion. Theabove-described invention is also applicable to a contact structure,comprising the nitride semiconductor for an electrode. Such a contactstructure is usable in a semiconductor device.

The present invention has an object of realizing a compoundsemiconductor having properties in a range partially overlapping that ofthe preferred numerical value range of the high concentration n-type GaNdisclosed in the above-mentioned PCT application and also having aresistivity in a range lower than that in the PCT application.

Embodiment 1 of the present invention provides a two-, three-, orfour-component compound semiconductor containing nitrogen and oneelement selected from the group consisting of B, Al, Ga and In, whichare group 13 elements. A combination of two property values of electronconcentration and resistivity fulfills numerical value conditionsenclosed by four points represented by:

(a) the electron concentration of 1.8×10²⁰ cm⁻³ and the resistivity of0.25×10⁻³ Ω·cm,

(b) the electron concentration of 3.6×10²⁰ cm⁻³ and the resistivity of0.25×10⁻³ Ω·cm,

(c) the electron concentration of 6×10²⁰ cm⁻³ and the resistivity of0.15×10⁻³ Ω·cm, and

(d) the electron concentration of 3×10²⁰ cm⁻³ and the resistivity of0.15×10⁻³ Slam.

Alternatively, embodiment 1 of the present invention provides a compoundsemiconductor fulfilling the numerical value conditions enclosed by thefour points mentioned in (a-1) to (d-1) mentioned above. Specifically,embodiment 1 of the present invention is directed to a nitridesemiconductor containing GaN as a main component.

An embodiment of the present invention regarding a manufacturing methodprovides a method for manufacturing a two-, three- or four-componentcompound semiconductor containing nitrogen and one element selected fromthe group consisting of B, Al, Ga and In, which are group 13 elements.The method includes:

pulsed-sputtering a target metal containing at least Ga in a chamber ina process atmosphere containing noble gas, nitrogen gas and oxygen toform a film of the compound semiconductor having a resistivity of0.4×10⁻³ Ω·cm or lower with a growth rate of 450 nm/h or less.

A compound semiconductor exhibiting desired property values inaccordance with the specifications or the use within the numerical valueconditions enclosed by (a) to (d) may be manufactured. It is easy tochoose the numerical value conditions from region X. For a use that doesnot require a compound semiconductor having a very low resistivity, acompound semiconductor matching the conditions of region X₂ may bemanufactured and used. In the case where a low resistivity isparticularly required, a compound semiconductor matching the conditionsof region X₁ may be manufactured and used.

INDUSTRIAL APPLICABILITY

The two-, three- or four-component nitride semiconductor according tothe present invention fulfills the numerical value conditions enclosedby (a) to (d) or (a-1) to (d-1) to exhibit a superb, namely, lowresistivity or a high electron mobility that has not been realized bythe conventional art.

The present invention is applicable to an important circuit element thatdetermines the performance of an electronic circuit, such as a contactportion of a wiring structure that is included in an electronic devicehaving a low electric resistance and requiring a large amount ofelectric current, for example, a horizontal or vertical powersemiconductor device such as a HEMT or the like, a high withstandvoltage diode, a thin film transistor, a display device or the like, anactive layer or the like.

The nitride semiconductor according to the present invention is usablefor a high-speed communication device, a computation device, a solarcell, a control circuit, an electronic device for au automobile or thelike in addition to the power semiconductor device, the display deviceand the light emitting device.

REFERENCE SIGNS LIST

-   -   1 Sputtering apparatus    -   2 Take-out roll    -   3 Take-in roll    -   4 Substrate film    -   5 Film formation chamber    -   10 Continuous film formation apparatus    -   11 Chamber    -   12 Substrate electrode    -   13 Sputter Source    -   14 DC power supply    -   15 Power supply controller    -   16 Nitrogen supply source    -   17 Heating device    -   12 a Heat dissipation sheet    -   21 Substrate    -   22 GaN    -   31 Substrate    -   32 GaN    -   33 Insulating layer    -   34 Insulating layer    -   35 Contact hole    -   41 n-type GaN contact layer    -   42 Ti layer    -   43 Al layer    -   44 Ni layer    -   45 Au layer    -   100 Vertical power MOSFET    -   101 Drain    -   102 n⁺-GaN layer    -   103 n⁻-GaN layer    -   104 p-GaN layer    -   105 n⁺-GaN layer    -   106 Insulating film    -   107 Source    -   108 Gate    -   200 LED    -   201 Substrate    -   202 n-type nitride semiconductor layer    -   203 Active layer    -   204 p-type nitride semiconductor layer    -   205 n-side electrode    -   206 p-side electrode    -   300 Schottky diode    -   301 n⁺-GaN layer    -   302 n⁻-GaN layer    -   303 Ohmic electrode    -   304 Schottky electrode    -   305 Insulating film    -   306 n⁺-GaN layer

The invention claimed is:
 1. A GaN semiconductor, wherein: the GaNsemiconductor contains Si and does not contain Ge as donor impurities,and a combination of two property values of electron concentration andresistivity fulfills numerical value conditions enclosed by four pointsrepresented by: (a) the electron concentration of 1.8×10²⁰ cm⁻³ and theresistivity of 0.25×10⁻³ Ω·cm, (b) the electron concentration of3.6×10²⁰ cm⁻³ and the resistivity of 0.25×10⁻³ Ω·cm, (c) the electronconcentration of 6×10²⁰ cm⁻³ and the resistivity of 0.15×10⁻³ Ω·cm, and(d) the electron concentration of 3×10²⁰ cm⁻³ and the resistivity of0.15×10⁻³ Ω·cm.
 2. The GaN semiconductor according to claim 1, whereinthe resistivity is 0.190×10⁻³ Ω·cm or lower.
 3. The GaN semiconductoraccording to claim 1, wherein the GaN semiconductor has an RMS value,obtained by a surface roughness measurement by an AFM, of 1.5 nm orless.
 4. The GaN semiconductor according to claim 1, wherein the GaNsemiconductor has an n-type conductivity and an electron mobility of 80cm²/(V·s) or higher.
 5. The GaN semiconductor according to claim 1,wherein the GaN semiconductor has an n-type conductivity and an electronmobility of 130 cm²/(V·s) or lower.
 6. A GaN semiconductor, wherein: theGaN semiconductor contains Ge and does not contain Si as donorimpurities, and a combination of two property values of electronconcentration and resistivity fulfills numerical value conditionsenclosed by four points represented by: (a) the electron concentrationof 1.8×10²⁰ cm⁻³ and the resistivity of 0.25×10⁻³ Ω·cm, (b) the electronconcentration of 3.6×10²⁰ cm⁻³ and the resistivity of 0.25×10⁻³ Ω·cm,(c) the electron concentration of 6×10²⁰ cm⁻³ and the resistivity of0.15×10⁻³ Ω·cm, and (d) the electron concentration of 3×10²⁰ cm⁻³ andthe resistivity of 0.15×10⁻³ Ω·cm.
 7. A contact structure, comprising aconductive portion containing the GaN semiconductor according to claim 1and an electrode connected with each other.
 8. A semiconductor device,comprising the contact structure according to claim
 7. 9. A transparentelectrode, comprising the GaN semiconductor according to claim
 1. 10.The GaN semiconductor according to claim 1, wherein the combination ofthe two property values of electron concentration and resistivityfulfills the numerical value conditions enclosed by the four pointsrepresented by: (a) the electron concentration of 1.8×10²⁰ cm⁻³ and theresistivity of 0.25×10⁻³ Ω·cm, (b) the electron concentration of3.6×10²⁰ cm⁻³ and the resistivity of 0.25×10⁻³ Ω·cm, (c) the electronconcentration of 6×10²⁰ cm⁻³ and the resistivity of 0.15×10⁻³ Ω·cm, and(d) the electron concentration of 3×10²⁰ cm⁻³ and the resistivity of0.15×10⁻³ Ω·cm, while excluding all combinations of the two propertyvalues enclosed by the four points that have an electron concentrationhigher than 3.95×10²⁰ cm⁻³.
 11. The GaN semiconductor according to claim6, wherein the combination of the two property values of electronconcentration and resistivity fulfills the numerical value conditionsenclosed by the four points represented by: (a) the electronconcentration of 1.8×10²⁰ cm⁻³ and the resistivity of 0.25×10⁻³ Ω·cm,(b) the electron concentration of 3.6×10²⁰ cm⁻³ and the resistivity of0.25×10⁻³ Ω·cm, (c) the electron concentration of 6×10²⁰ cm⁻³ and theresistivity of 0.15×10⁻³ Ω·cm, and (d) the electron concentration of3×10²⁰ cm⁻³ and the resistivity of 0.15×10⁻³ Ω·cm, while excluding allcombinations of the two property values enclosed by the four points thathave an electron concentration higher than 5.49×10²⁰ cm⁻³.